Water Treatment

water purification | ferrous sulphate | watercare | coagulation experiment

Water Purification

Collection. Water has to be got from somewhere. A dam is often used to make a water reservoir to collect rain water, or an intake in a river can be used to collect river water.
Screening. Branches, leaves and other rubbish are removed using a coarse filter.
Coagulation/flocculation. When particles are small enough they stay in suspension because a tiny negative charge on them holds them apart and keeps them from settling. Aluminium sulphate (Al₂(SO₄)₃) or ferrous sulphate (iron(II) sulphate, FeSO₄) is added to water to make the tiny particles stick together in lumps called "floc".

Aluminium sulphate is often called alum, although alum is actually a family of compounds with the general formula AM(SO₄)₂·12H₂O where A is a monovalent cation such as potassium or ammonium, and M is a trivalent metal ion such as aluminium or chromium.
Sedimentation. Heavy particles settle out, leaving clear-looking water.
Filtration. The water is passed through various grades of sand filters to remove any particles left. Water from the Waikato River is also filtered with charcoal, to make sure extra fine particles are removed.
Disinfection. Chlorine is added to kill bacteria and micro-organisms, and prevent extra growing in the future.
Fluoridation. In all Auckland water except the water in Onehunga (which comes from a spring) and some other parts of New Zealand the naturally occuring fluoride level is adjusted (normally up, sometimes down) to between 0.7 ppm and 1.0 ppm. Some people don't like fluoride in drinking water, on the basis that fluorine is a poisonous element. This is irrelevant for the levels of fluoride used (and note why chlorine is used in the previous point above). Some people argue that it is mass medication, and is immoral. Consider: Fluoridation in New Zealand costs about 50 cents per person per day and results in about 40% less tooth decay in the areas it's used. Is it moral to not use it?
Storage. Water is pumped into large water tanks at high points around the city. Because Auckland is so hilly (it's built on 50 volcanos) tanks are often placed on the top of hills or extinct volcanos. For example, at the intersection of Richardson Rd and Hillsborough Rd (next to Hillside BP) or on the top of Big King (the last of the Three Kings remaining). There is also a water tank in the top of Mt Roskill – the flat area at the top used to be the crater.

Ferrous Sulphate

In chemistry the word ferrous refers to iron in its +2 state, Fe²⁺, while ferric refers to iron in its +3 state, Fe³⁺. "Ferrous" can also be used in other non-chemical ways. For example, ferrous metals are metals that contain iron, such as wrought iron, steel, etc.

Ferrous sulphate, or iron(II) sulphate (note the spacing) is another molecule which is normally hydrated with extra water molecules. There are several hydrates.

FeSO₄·H₂O – colourless.
FeSO₄·4H₂O – white.
FeSO₄·5H₂O
FeSO₄·7H₂O – blue-green, loses 6 water molecules at 90 °C to become the monohydrate. With further heating, at 480 °C FeSO₄ decomposes to red-brown Fe₂O₃ and produces SO₂ and SO₃, the latter of which is nasty. Do not heat it that much!

FeSO₄ is useful for water treatment as it is an effective flocculation and phosphate removal, and is less expensive than Al₂(SO₄)₃. However, it has a couple of side effects – it can make the water taste bad, and it can leave a brown stain on white porcelain. Having a little iron in water after treatment isn't completely bad though, as it can help prevent anaemia.

Ferrous sulphate is also used to colour concrete yellow to rusty brown, and as a lawn conditioner.

ferrous sulphate heptahydrate

In the Middle Ages ink for writing was made from ferrous sulphate and tannin, and was a purple-black colour that adhered well to vellum writing surfaces, as it made chemical bonds with the proteins in the vellum.

Watercare Services Ltd

WaterCare Services Ltd provides the water in Auckland. They have a page explaining the basic process they use for water from the Waitakere and Hunua Ranges (punctuation corrected in the first paragraph):

Raw water coming into the treatment plants from the Hunua and Waitakere dams is exceptionally clean, from only 2-5 ntu (ntu is a measurement of turbidity or presence of dirt particles). This is because there is almost no human, industrial or agricultural activity in those areas. Raw water from the Waikato River has a higher turbidity (20-40 ntu), however the sophisticated 4-stage treatment process means that the water, once treated, is as clean if not cleaner than water supplied from the other treatment stations.

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The standard 3-stage treatment process followed at all of Watercare's treatment stations, with the exception of Waikato, is as follows:

Coagulation
Coagulation destabilises the predominantly negatively charged particles suspended in the water. By lessening their repelling qualities the particles attach to each other for easier removal later on. The coagulant added to the raw water is aluminium sulphate.

The water then flows through mixing tanks where the particles increase in size, to create what's known as floc. This floc settles to the bottom of the tanks. Positively charged polyelectrolyte molecules and alkaline lime are also added to aid the coagulation process.

Clarification and filtration
The floc is removed at this stage in vertical flow clarifiers shaped like inverted pyramids. This process produces a blanket of floc, above which clarified or settled water decants into troughs at the top of the tanks. Floc, now called clarifier sludge, is drained off from the blankets through cones suspended at the tops of the clarifiers.

The settled water is then fed through to sand filters to further remove suspended solids. The filters are washed every 24-48 hours to clean out entrapped floc.

Disinfection
Either gaseous chlorine or sodium hypochlorite is added to kill any pathogenic bacteria that might be in the water after filtration. Watercare produces its own sodium hypochlorite on site from electrolysis of common salt. The amount of chlorine in the water that leaves the stations is about 0.9 grams per cubic metre.

So that answers another question we had – how much chlorine is in tap water – just under 1 ppm. This compares with swimming pool water which should have 2-4 ppm (more if it's a small pool), and spa pools about 10 ppm – the smaller the pool the greater the effect of swimmers on available chlorine levels. See pool shocking for more information about how to maintain swimming pool chlorine levels.

For water stored as part of a civil defence kit, to be ready to drink in an emergency it is important that some chlorine is in the water, so distilled water should not be used. It should be tightly capped and kept in a cool, dark place, and replaced about every six months. Without this care water will likely grow algae, which means that it'll need to be boiled before being drunk.

WaterCare also have a page explaining how water from the Waikato River is treated.

Coagulation Experiment

Ferrous sulphate crystals were added to muddy water to make the mud settle out faster. See the Coagulation Experiment.